A 40AH battery is next to useless unless you have much smaller than normal loads.
There is lots of info out there about solar power. Basically, your decisions on what you need hinges on:
- how many amp hours per day do you normally use (need to replenish)? Often, parasitic draws of a camper total around 24AH per day (or more). Add in 12V lighting (LED lighting makes a significant difference) and fans (furnace fan is often 5 amps when running).
- can you live without AC (120V) power (a lot more solar panel, battery bank, and inverter to run AC stuff like air conditioning, microwave, etc.)
- how big a solar panel(s) do you need to produce the needed power where you will be camping? How much direct sunlight per day can you expect at that location?
Based on your answers, there are guidelines for sizing the solar panel(s), battery bank, controller, and so on.
Insufficient sizing of components will likely result in batteries running low and dying prematurely - battery life is dependent on how many discharge/recharge cycles and how deeply the battery is discharged.
just my thoughts and experiences